The recognition of microbes by the innate and adaptive immune system can lead to the resolution of infection and development of long-lived immunity. Many microbial pathogens gain entry to the host by penetrating mucosal surfaces of the lung, intestine and genito-urinary tract. However, the induction of immune responses to microbial pathogens at mucosal surfaces is not well understood. The specific aims of the proposal are: Aim 1. To identify the cell types that present Salmonella antigens in vivo in order to test the hypothesis that lymphoid dendritic cells activate Salmonella-specific CD4 T cells. These studies will directly examine the presentation of a Salmonella encoded antigen in vivo, in order to test the hypothesis that lymphoid dendritic cells are responsible for activating Salmonella-specific CD4 T cells after oral infection. Aim 2. To examine Salmonella-specific T cell activation in the spleen and define mechanisms that account for T cell unresponsiveness in this organ. Our preliminary data indicate that Salmonella-specific CD4 T cells are inefficiently activated in the spleen, despite bacterial replication in this organ. We hypothesize that the location of bacteria in the spleen red pulp physically separates antigen from Salmonella-specific T cells. This will be tested using a novel Salmonella-specific TCR adoptive transfer system. Aim 3. To examine CD4T cell differentiation and migration in order to test the idea that effector/memory T cells are not efficiently generated after Salmonella infection. Our preliminary data indicate a defect in non-lymphoid migration of Salmonella-specific T cells following oral infection. We hypothesize that T cell effector functions do not develop efficiently, due to the local mucosal priming environment. This issue will be tested by examining the effector cytokine production and non-lymphoid migration of Salmonella-specific T cell after oral infection. These studies will provide new insight into the development of immunity to mucosal pathogens.